زیست چینه نگاری سازند دلیچای در برش پریخان (غرب شاهرود) بر مبنای پالینومورف‌ها و روزن‌داران

سازند دلیچای با سنگ شناسی شیلی مارنی و با میان لایه‌های سنگ آهکی به ضخامت 250 متر در برش پریخان در غرب شاهرود به منظور مطالعات زیست چینه نگاری مورد بررسی قرار گرفت. در این مطالعه 30 گونه متعلق به 22 جنس داینوفلاژله، 10 گونه متعلق به 8 جنس اسپور و 7 گونه متعلق به 4 جنس پولن شناسایی شد. ‌‌‌‌جهت تعیین سن و انجام زیست چینه نگاری از حضور فراوان و مستمر میوسپور klukisporites variegatus و پولن callialasporites dampieri که از گونه‌های شاخص ژوراسیک میانی می‌باشند و وجود داینوفلاژله‌های شاخص زیست‌زون‌های باژوسین پسین مانند cribroperidinium crispum total range zone، افراد شاخص باتونین کالووین پیشین در dichadogonyaulax sellwoodii interval zone (کالووین پیشین میانی) و گونه‌های شاخص ctenidodinum continuum interval zone و همچنین زیست‌زون‌های مربوط به روزن‌داران کف‌زی و شناور شامل lenticulina variansophtalmidium assemblage zone و globigerina bathoniana taxon range zone که تایید کننده سن باژوسین پسین تا کالووین میانی است، استفاده شد. مقایسه داینوسیست‌ها و روزن‌داران سازند دلیچای با میکروفسیل‌های معرفی شده در ایران، آسیای شرقی، استرالیا، آفریقا، آمریکای جنوبی و اروپا حکایت از تطابق نسبی با این مناطق دارد.

Biostratigraphy of Dalichai Formation in Parikhan section in the west of Shahrood, based on palynomorphs and foraminifera

IntroductionThe Dalichai Formation in Parikhan section in the west of Shahrood with a thickness of 250 meters consisting of an alternation of bluishgray was studied. This formation overlies the Shemshak formation disconformity, and it has gradually been covered by the thickbedded limestones of the Lar formation. The biogenic components consist of brachiopods, gastropods, bivalves, ammonites, echinoderms, belemnites, sponges and sponge spicules, radiolarians, ostracods, benthic foraminifers. The aim of the present study is to discuss the age of the formations and paleobiogeography relationship based on foraminifera and dinoflagellate cyst assemblage. Materials and MethodsThe Parikhan section is located on the central part of Alborz Mountains, approximately 14 km west of Shahrood (coordinates with E 54° 49΄11 ˝ and N 36° 24΄20˝). 27 samples were collected systematically and processed for palynostratigraphy following the standard palynological technique described in Traverse (2007). Cold hydrochloric (30 %) and hydrofluoric acids (30 %) were used to dissolve carbonate sand silicates, respectively. The residue was neutralized and centrifuged in ZnCl2 with specific gravity (1.9 g/cm3). The materials were then sieved using a 15μm nylon mesh and mounted on microscope slides using liquid Canada balsam. The present study shows that the Dalichai Formation is rich in terrestrial (spores and pollen grains) and marine palynomorphs (dinoflagellate cysts, acritarchs, and scolecodonts). In the laboratory, thin sections were prepared from all marly limestones and hard resistant shaly marl samples to achieve further microfossils evidence and aid sedimentological descriptions. All samples were processed for foraminiferal analysis following the standard method of Lirer (2000). Thus, 21 species belonging to 17 genera of planktonic and benthonic foraminifera were identified. Discussion and ResultsIn the present study, 30 species of dinoflagellate cysts belong to 22 genera and 10 species of spore, which belong to 8 genera, and 7 species of pollen, which belong to 4 genera, have been identified. Based on the presence and stratigraphic distribution of the most dominant miospore, Klukisporites variegatus and Callialasporites dampieri of pollen are identified in the Dalichai Formation (late Bajocian Callovian). Based on recorded dinocyst assemblages, especially marker species, three palynozone, including Cribroperidinium crispum Total Range Zone (late Bajocian), Dichadogonyaulax sellwoodii Interval Zone (Bathonian – early Callovian), and Ctenidodinium continuum Interval Zone (late Callovian) were identified.Cribroperidinium crispum Total Range ZoneThis biozone encompasses 43 m of the base of the Dalichai Formation from samples 1 to 9. The zone differentiated here is equivalent to the Cribroperidinium crispum zone of Poulsen and Riding (2003) for subboreal Northwest Europe and Cribroperidinium crispum zone of Mafi et al. (2013) and GhasemiNejad et al. (2012) for Northern Iran. Proximate dinocyst such as Nannoceratopsis spp., Nannoceratopsis pellucida, Pareodinia ceratophora are abundant in the zone. Meiourogonyaulax valensii is also relatively common in this biozone.Dichadogonyaulax sellwoodii Interval ZoneThe Dichadogonyaulax sellwoodii zone (Riding and Thomas 1992) is equivalent to the Ctenidodinium sellwoodii zone of Poulsen and Riding (2003) for subboreal Northwest Europe and the Dichadogonyaulax sellwoodii zone of Mafi et al. (2013) and GhasemiNejad et al. (2012) for Northern Iran. This biozone encompasses 57 m of the Dalichai Formation above the Cribroperidinium crispum zone (samples 9 to 18). Dichadogonyaulax sellwoodii and Ctenidodinium combazii have their first appearances at the base of this zone. Many proximate and proximochorate dinoflagellate cysts such as Ctenidodinium combazii and Ctenidodinium spp. were recorded in this zone. These are considered to indicate an age of Bathonian to Early Callovian for this zone.Ctenidodinium continuum Interval ZoneThis zone encompasses 39 m of the Dalichai Formation above the Dichadogonyaulax sellwoodii zone (from samples 19 to 25) and is marked by the LAD of Ctenidodinium combazii. A slight change has been recorded in the upper boundary of the zone as the index species Scriniodinium crystallinium whose FAD marks the end of the zone in Britain (Riding and Thomas 1992), subboreal Northwest Europe (Poulsen and Riding 2003), and Northern Iran (Mafi et al. 2013) has not been recorded. We, therefore, used the FAD of Ctenidodinium tenellum as the marker for the upper boundary of this zone. These indicate the middle to late Callovian for this zone. Dinoflagellate species are abundant, but all the species were concentrated in lower parts of the Dalichai Formation in the Parikhan section; therefore, planktonic and benthic foraminifera was used to determine exact age. The planktonic foraminifera comprise approximately 5–10% of the assemblage at the studied locality, and are found within a benthic foraminiferal assemblage consisting of a mixture of smaller agglutinated species and calcareous species (nodosariids, opthalmidiids and spirillinids) without any larger foraminifera. Planktonic foraminifer’s preservation varies from poor to moderate. The planktonic foraminifera association from the Parikhan section has been compared with those previously reported from time equivalant strata worldwide (Boudagherfadel et al., 1997). This formation was divided into two biozones based on Lenticulina variansOphtalmidium Assemblage zone and Globigerina bathoniana TaxonRange Zone. In the early Bathonian, the beds show the first appearance of planktonic foraminifera. Two protoglobigerinid genera are recognized: a lowspired species identified as Globuligerina sp. and a highspired form assigned to Conoglobigerina sp. Our discovery is the first report of planktonic foraminifera in the Middle Jurassic (lower Bathonian) of Iran. It can be correlated throughout the Middle East, from Saudi Arabia to Oman and Syria. The assemblages recorded from the Dalichai Formation are very similar to coeval Northeast Iran and Northwest European populations. The close similarities of dinoflagellate cyst assemblages and planktonic foraminifera recorded from the Dalichai Formation with Northwest European populations confirm the connection between two sedimentary basins during Middle Jurassic. ReferencesBouDagherFadel, M.K., Banner, F.T., Whittaker, J.E., 1997. Early Evolutionary History of Planktonic Foraminifera. British Micropalaeontological Society Publication Series, Chapman and Hall, 269p.GhasemiNejad, E., Sabbaghiyan, H., Mosaddegh, H., 2012. Paleobiogeographic implications of late Bajocianlate Callovian (Middle Jurassic) dinoflagellate cysts from the Central Alborz Mountain, northern Iran. Journal of Asian Earth Sciences, 43: 110.Lirer, F., 2000. A new technique for retrieving calcareous microfossils from lithified lime deposits. Micropaleontology, 46: 365–369Mafi, A., GhasemiNejad, E., Ashouri, A.R., Vahidinia, M., 2014. Dinoflagellate cysts from the Upper BajocianLower Oxfordian of the Dalichai Formation in Binalud Mountains (NE Iran): their biostratigraphical and biogeographical significance. Arabian Journal of Geosciences, 7: 3683–3692Poulsen, N.E., Riding, J.B., 2003. The Jurassic dinoflagellate cyst zonation of Subboreal Northwest Europe. In: Ineson, J.R., Surlyk, F., (eds.), The Jurassic of Denmark and Greenland. Geological Survey of Denmark and Greenland Bulletin, 1: 115144.Riding, J.B., Thomas, J.E., 1992. Dinoflagellate cysts of the Jurassic System. In: Powell, A.J., (ed.), A stratigraphic index of dinoflagellate cysts. British Micropalaeontological Society Publications Series. Chapman and Hall, London: 797.Traverse, A., 2007. Paleopalynology: Topics in Geobiology, second edition. Springer, Dordrecht, Netherlands, 813 p.